WO2010069287A2

WO2010069287A2 - Method for the deposition of microcrystalline silicon on a substrate

Info

Publication number: WO2010069287A2
Application number: PCT/DE2009/001649
Authority: WO
Inventors: Aad Gordijn; Thilo Kilper; Bernd Rech; Sandra Schicho
Original assignee: Forschungszentrum Jülich GmbH
Priority date: 2008-12-18
Filing date: 2009-11-18
Publication date: 2010-06-24
Also published as: DE102008063737A1; EP2368275A2; WO2010069287A3; CN102257630A; JP5746633B2; DE102008063737A9; US20110284062A1; JP2012512534A

Abstract

The invention relates to a method for depositing microcrystalline silicon on a substrate in a plasma chamber system, comprising the following steps: the plasma chamber system contains at least one reactive, silicon-containing gas and hydrogen or exclusively hydrogen before the plasma is initiated; the plasma is initiated; exclusively reactive, silicon-containing gas or at least one mixture comprising a reactive, silicon-containing gas and hydrogen is continuously fed to the chamber system once the plasma has been initiated, the concentration of reactive, silicon-containing gas that is fed to the chamber being set at more than 0.5 percent; and the plasma performance is adjusted to within a range of 0.1 and 2.5 W/cm² of electrode surface, a deposition rate exceeding 0.5 nm/s is selected, and the monocrystalline layer is deposited on the substrate at a thickness of less than 1000 nanometers.

Description

Method for depositing microcrystalline silicon on a substrate

The invention relates to a method for depositing microcrystalline silicon on a substrate.

Microcrystalline silicon (μc-Si.Η) is a material that is used in particular in solar cells as an absorber material. Today, it is produced in many laboratories by means of the PECVD process (plasma-enhanced chemical vapor deposition) from silicon-containing gas (typically silane) and hydrogen.

High quality layers of microcrystalline silicon can be deposited in different deposition regimes. Typically, the PECVD

Method, the silane / hydrogen mixture is admitted into the plasma chamber and a corresponding amount of gas is pumped while simultaneously burning in the plasma chamber, a plasma which causes the dissociation of silane and hydrogen molecules and thus the generation of precursors for the growing microcrystalline silicon layer , When depositing with PECVD at the standard industry frequency of 13.56 MHz, the silane concentration (= silane flux / (sum of silane flux and hydrogen flux)) is typically about 1%, at excitation frequencies in the VHF range typically less than 10%. The hydrogen is needed to influence the layer growth. However, only a small portion of the hydrogen used is incorporated into the produced silicon layer, typically less than 10%. The remaining hydrogen is pumped out.

Methods for depositing microcrystalline silicon by means of PECVD are known from the prior art, which can reduce hydrogen consumption, which is a factor in solar cell manufacturing costs.

1. The closed-chamber CVD process (CC-CVD). This investigated process runs cyclically (discontinuously) and essentially comprises two process steps. In a first step, a small amount of the reactive process gas (silicon-containing gas, z. SiH ₄ or a ClVSiΗU mixture) in a ratio of about 25% reactive gas to hydrogen through the chamber. This step serves to refresh the gas atmosphere after a process cycle. During this time, the plasma burns at low power (about 10 W), so that an ultra-thin silicon layer is deposited. In the following second step, both the pumping power from the chamber and the gas supply into the chamber are interrupted. By delaying the shutdown of the hydrogen supply, the deposition pressure is increased and the silane concentration is lowered to ~ 5%. The plasma now burns at about 60 W. The process gases are gradually decomposed and the deposited layer continues to grow. At the same time, an opposite effect occurs. The layer is etched by H radicals. The etch rate continues to increase due to the increasing proportion of hydrogen in the plasma, until finally a balance between layer growth and etching is achieved. Atoms that are weakly bound are preferentially etched, eventually forming a network with stronger bonds. The entire deposition process takes place as a continuous sequence of these two steps (layer by layer) to the desired layer thickness. It is reported by a crystalline volume fraction of more than 90%. Due to the cyclic change of the process conditions, this process is very expensive. It differs fundamentally from the standard PECVD method, and is not yet suitable for use in industry. So far no solar cell has been realized with this method.

2. The "Static closed chamber procedure" (VHFGD). This "very high frequency glow dis- charge" (VHFGD) deposition process is a continuous process without any use of hydrogen. The deposition chamber (plasma chamber) is not completely isolated. A small silane flow is admitted into the chamber and an appropriate amount of gas is simultaneously pumped out. Deposition takes place at VHF excitation and at low pressure (0.1 mbar). In the deposition of silicon from silane is hydrogen-free. The low silane gas flow ensures that silane depletion occurs. The initial rapid deposition of silicon is due to the increase in dissociated! Hydrogen braked. After about one minute, static conditions with a small ratio of [SiH *] / [Hα] prevail, allowing for continuous microcrystalline growth. Since the deposition is started with a pure silane plasma and only added later by the decomposition of the silane hydrogen, the deposited layer has a pronounced amorphous incubation layer (~ 0.1 nm) as a first layer. This may be the case with the use of such layers in components, in particular in solar cells cause a significant impairment of the function. Thus, in solar cells whose absorber layer was produced by this method, only an efficiency of 2.5% could be achieved.

3. DE 103 08 381 A1 discloses a method for depositing microcrystalline silicon for the absorber layer, which likewise has low hydrogen consumption. For this purpose, a lot of hydrogen is started, and after the plasma start the hydrogen supply is radically reduced. In contrast to methods 1 and 2, however, it is possible with this method to achieve significantly higher solar cell efficiencies of up to 7%.

For the industrial applicability of a process for the production of solar cells, in addition to the hydrogen consumption, the quantities produced per unit time are an important criterion. For a given deposition rate in nm / s, therefore, the thickness of the solar cell is a criterion.

From Vetterl et al. (O. Vetterl, A. Lambertz, A. Dasgupta, F. Finger, B. Rech, O. Kluth, H. Wagner (2006).) Thickness dependence of microcrystalline silicon solar cell properties., Solar Energy Materials & Solar Cells. 345-351), it is known that for layer thicknesses <1 μm for the μc-Si: H absorber layer (i-layer) the achievable efficiency η of μc-Si: H single solar cells as the product of open circuit voltage (Voc), short-circuit current density (Jsc) and Fill factor (FF) decreases significantly. In practice, therefore, a layer thickness of> 1 μm has become established as the optimized solar cell for the μc-Si: H absorber layer. This provides single solar cells with an efficiency greater than or equal to 8% when using a ZnO / Ag back contact. As a back contact, the side facing away from the light is called. As a front contact, the light-facing side is called.

From Rath (JK Rath (2003). Low temperature polycrystalline silicon: Solar Energy Materials & Solar Cells 76, 431-487) it is known that the parameter deposition rate of μc-Si: H absorber layer also adversely affects the parameter efficiency of the subsequent cell. The higher the deposition rate for the absorber layer, the lower the achievable efficiency of the cell. The increase in the deposition rate for the μc-Si: H absorber layer, for example by increasing the plasma power to more than 0.3 W / cm ² electrode surface, thus disadvantageously leads to an excitation frequency of the plasma of 13.56 MHz in μc-Si: H single solar cells with standard layer thicknesses of about 1000 to 2000 nanometers, the achievable cell efficiency already decreases significantly. The same applies correspondingly to the stacked solar cells comprising such μc-Si: H absorber layers.

The object of the invention is to provide a rapid method for the deposition of microcrystalline silicon on a substrate, which nevertheless leads to high efficiencies of the solar cells.

The object is achieved by a method according to claim 1. Advantageous embodiments emerge from the claims referring back to this.

The method for depositing microcrystalline silicon on a substrate in a plasma chamber system comprises the steps of:

- Before the start of the plasma, the plasma chamber system has a reactive, silicon-containing gas and hydrogen or exclusively hydrogen, the plasma is started, the chamber system is continuously fed exclusively reactive, silicon-containing gas after the plasma start or the chamber system is after the plasma continuously adding a mixture comprising a reactive silicon-containing gas and hydrogen, wherein the concentration of reactive, silicon-containing gas is set greater than 0.5% upon delivery to the chamber, and

the plasma power is set between 0.1 and 2.5 W / cm ² electrode surface, and a deposition rate of greater than 0.5 nm / s is selected and a microcrystalline layer is deposited on the substrate, which has a thickness of 1000 nm does not exceed.

The silane concentration (= silane flow / (sum of silane flow and hydrogen flow)) is adjusted so that the deposition of the microcrystalline absorber layer takes place close to the μc-Si: H / a-Si: H growth transition. The reason for this is that optimum μc-Si: H absorber layer properties are not achieved by a high crystalline content, but that materials in the transition region with amorphous and crystalline fractions show the best properties. The cause of these favorable properties is considered to be an optimal interfacial passivation of the silicon crystallites by amorphous silicon. For the deposition of the microcrystalline layer, the process gases (silicon-containing gas and hydrogen) before and / or after the plasma start inert gases, such as argon, are added.

An increase in the generator power leads to a higher crystalline volume fraction of the growing layer, with otherwise constant process parameters.

The method according to the invention advantageously has the features which are particularly advantageous for mass production of solar cells on an industrial scale. This is the deposition of a comparatively thin μc-Si: H absorber layer with simultaneously high deposition rate. It is irrelevant whether the microcrystalline absorber layer is deposited for a single cell or for a stacked solar cell. Only the measure of increasing the deposition rate in thin microcrystalline absorber layers solves the object of the invention, regardless of the substrate selected and the associated structure of the solar cell.

The term plasma chamber system includes all common plasma chambers for the deposition of solar cells. The method can therefore basically be used for single chambers as well as for combined plasma chamber systems of several single chambers.

The plasma chamber system can, for example, be a single chamber for the production of intrinsic a-Si: H and μc-Si: H absorber layers (i-chamber), a further single chamber for the production of p- or n-doped a-Si: H and μc Si: H layers (doping chamber) and a loading chamber for the insertion and removal of substrates include.

In the context of the invention, it was recognized that with an increase in the deposition rate to greater than 0.5 nm / s and at the same time thin μc-Si: H absorber layers having a layer thickness of less than 1000 nm, no appreciable loss of efficiency due to a deposition rate increase takes place any more. All combinations of intermediate values for the deposition rate and the layer thickness are allowed. In contrast to this, in the thick microcrystalline absorber layers preferred in the state of the art with more than 1000 nm layer thickness, a deposition rate increase, for example from 1 to 2.5 nm / s, leads to a significant loss of efficiency. The deposition rate can be selected up to 5 nm / s depending on the plasma power or generator power.

To produce p- or n-doped silicon layers in the doping chamber, a boron-containing (for example trimethylboron B (CH ₃ ) ₃ ) or phosphorus-containing gas (for example phosphine PH ₃ ) can additionally be mixed with the process gas mixture comprising silicon-containing gas and H ₂ , In order to avoid dopant carry-over in the production of intrinsic silicon absorber layers as far as possible, the i-chamber and the doping chamber are preferably separated from one another via a lock gate.

The i-chamber may include one or more different high-frequency electrodes, which may have a "showerhead" design for homogeneous gas injection, and whose electrode surface may be vertically or horizontally aligned.The substrate to be coated may be moved to the electrode by means of a transport system Once the substrate has reached its final position, a heater system ensures that the substrate is heated to the desired substrate temperature before deposition begins, for example, the distance between the radio frequency electrode and the substrate can be set between 5 and 25 mm ,

The term deposition rate is defined as the thickness of the deposited absorber layer per unit time. The deposition rate increases in principle with the increase in plasma power or generator power.

Preferably, a plasma excitation frequency of 13.56 to about 100 MHz with an electrode spacing of 5 to 25 millimeters, preferably 10 to 25 millimeters can be selected.

During the process, the deposition pressure in the plasma chamber is preferably set between 1 and 25 mbar in the plasma chamber or in the plasma chambers. An increase in the deposition pressure leads, with otherwise constant process parameters, to a lower crystalline volume fraction of the growing layer.

The method may be conducted, so in a further, particularly advantageous embodiment of the invention is that the base pressure ^{"is 6} mbar or even more, that is to say z. B. 10 ^'5 mbar or z. B. ^10" in the plasma chamber system at least 10 ⁴ mbar. All Intermediate values are possible.

The base pressure is the prevailing pressure in the plasma chamber or in the plasma chamber system in the evacuated state prior to the introduction of the process gases and depends inter alia on the design of the chamber and the purity of the gases used.

According to the prior art, the plasma chamber or the plasma chamber system of a small laboratory plant has a base pressure of about 10 ^-8 mbar, which is due to the high levels of purity under which the prior art processes must be operated in order to achieve the desired high levels Thus, the realization has become established that the oxygen content in μc-Si: H absorber layers should not exceed a certain limit, otherwise the achievable efficiency of both μc-Si: H based single solar cells and a-Si H / μc-Si: H-based stacked solar cells experienced significant and undesirable losses in efficiency In terms of process purity was to ensure according to the prior art accordingly, that the oxygen content in the μc-Si: H absorber layer always remained below an allowable limit to achieve high cell efficiencies, this limit According to the prior art, the μc-Si: H absorber ^{layer contains} about 2 ^× 10 ¹⁹ / cm ³ oxygen atoms and standard ^{layer thicknesses} of> 1000 nm for the μc-Si: H absorber layer. This was associated with high plant costs in terms of the degree of process purity due to the pump systems to be used. The operating costs as such are also comparatively high, since the purity of the process gases, essentially silane or silicon-containing gas in general and hydrogen had to be high.

In the context of the invention, it has been recognized that a synergistic effect occurs particularly at high deposition rates of greater than 0.5 nra / s and simultaneously small layer thicknesses of less than 1000 nm of the deposited microcrystalline absorber layer, in such a way that despite a very high base pressure of up to 10 ^'6 mbar, 10 ^"5 mbar or more, only a slight loss of efficiency of the cell occurs, which is absolutely tolerable from a process engineering point of view, with respect to these three parameters, deposition rate> 0.5 nm / s , Layer thickness <1000 nm and base pressure> 10 ^"6 mbar, in turn, each intermediate value can be assumed. It has been found that high deposition rates according to the invention, in comparison to low deposition rates, cause smaller quantities of oxygen to be incorporated into the growing μc-Si: H layer at the same base pressure. Furthermore, it was recognized that the tolerable oxygen content in μc-Si: H single solar cells, up to which no efficiency losses still occur, is higher for thin absorber layers than for thick absorber layers. The same applies again to all forms of stacked solar cells.

The consequence of the described discoveries, ie the influence of the deposition rate and / or the oxygen content on the achievable μc-Si: H single solar cell efficiency as a function of the absorber layer thickness, is that the production cost optimum for thin-film solar modules, the μc-Si : H absorber layers include, shifts to significantly lower overall layer thicknesses. This also applies to stacked solar cells.

It was recognized that in the context of the method according to the invention then the achievable cell efficiency possibly slightly decreases, but this disadvantage in addition to the shorter production time due to the layer thickness reduction and possibly lower claims on the purity of the process (the gases) and the cells produced in relation to the Oxygen content is overcompensated, and that in this case, the solar cell production time due to an increase in the deposition rate of the μc-Si: H absorber layer can be significantly reduced again without additional loss of efficiency. The production can thus be carried out with significantly lower requirements for the process purity, which brings significant cost advantages, namely lower system costs and lower process gas costs. In addition, it should be noted that thinner a-Si: H / μc-Si: H based stacked solar cells degrade less. As a result, the difference in the stabilized efficiency against a-Si: H / μc-Si: H based stacked solar cells having the standard total layer thickness becomes smaller than that at the initial efficiency.

The method can be carried out in a further embodiment of the invention so as to regulate the flows of the gases supplied to the chamber and derived from the chamber or gas mixtures such that forms a constant deposition pressure during the process. In particular, a deposition rate between about 1.0 to 2.5 nm / s can be selected and a microcrystalline layer with a thickness of 200 to 800 nm, in particular from 400 to 600 nm, can be deposited. In these cases, μc-Si: H single solar cells with an efficiency of about 7-8% can also be regularly realized with the mentioned method for all combinations of intermediate values. For a-Si: H / μc-Si: H based stacked solar cells, which initially degrade light-induced due to the a-Si: H content and then have a stabilized efficiency, higher initial efficiencies of about 9-11% and higher stabilized efficiencies of about 8-10% can be achieved easily.

After the plasma start, the chamber can be fed continuously only with reactive, silicon-containing gas in a volume flow of 0.5 sccm to 20 sccm / 100 cm ² coating area, in particular from 0.5 sccm to 10 sccm / 100 cm ² coating area , This applies z. Example, if the chamber or the chamber system before starting the plasma start has hydrogen without silicon-containing gas. At the same time, the gas mixture present in the chamber can be at least partially diverted from the chamber. Inert gases may be added to this process.

The method for depositing the microcrystalline silicon can be used in particular for the production of stacked solar cells. As a substrate in the process then z. B. an electrical contact layer and then deposited microcrystalline n-layer. The solar cell in the case of a tandem configuration has a nipnip configuration in its finished state. As an electrical contact layer z. As a metal ZnO layer or a metal-Snθ ₂ layer or another TCO layer can be selected. As the electrical contact layer of the nipnip layer alone, a metal layer can be selected as a reflector. Single cells are manufactured without the corresponding a-Si: H content.

However, a glass-TCO-a-Si.Η-layer system plus a microcrystalline p-layer deposited thereon can also be selected as the substrate, for example. It is also possible to choose a metal-TCO-a-Si.Η-layer system plus a microcrystalline p-layer deposited thereon. The solar cell has a pin-pin configuration in its finished state in these cases. For single cells, substrates without the corresponding a-Si: H content are selected. Of course, other substrates for the production of Stacked solar cells are used.

The solar cells produced by this method thus have at least one n-i-p structure or one p-i-n structure, wherein the absorber layer is microcrystalline. For example, μc-Si: H single solar cells, which are produced with this inventive method at a particularly advantageous deposition rate of 2.5 nm / s, an efficiency of about 7-8%. They are characterized in that the μc-Si: H absorber layer has a thickness of less than 1000 nanometers.

Particularly advantageous are already produced by the process according to the invention

Single solar cells have an efficiency of 7-8% despite high deposition rates and low layer thicknesses. In stacked solar cells correspondingly higher efficiencies can be achieved.

In a particularly advantageous embodiment of the invention, the solar cells produced by this method can have more than 2 * 10 ¹⁹ to about 1 * 10 ²¹ cm ^"3 oxygen atoms in the microcrystalline absorber layer.

An a-Si: H / μc-Si: H-based stacked solar cell particularly advantageously has a total layer thickness of all absorber layers of less than 1000 nanometers. The microcrystalline absorber layer preferably has an oxygen content of more than 2 × 10 ^19, in particular up to 10 ²¹ oxygen atoms / cm ^3, preferably with respect to the production costs per watt of peak solar module nominal power.

With the method, it is particularly advantageously possible to produce thin a-Si: H / μc-Si: H stacked solar cells, in particular tandem solar cells, the usual negative effects of a high deposition rate for the μc-Si: H absorber layer as well as a high concentration of contaminants in the μc-Si: H absorber layer are significantly less pronounced. This provides additional cost savings by further reducing manufacturing times and reducing process purity requirements. In addition, a synergistic effect occurs here, since high deposition rates in comparison with low deposition rates have the effect that, with the same base pressure, smaller amounts of oxygen are incorporated into the growing μc-Si: H layer. It has also been recognized within the scope of the invention that the relative degradation-related loss of efficiency in stacked solar cells with lower absorber layer thickness is less pronounced.

In addition, the invention will be explained in more detail with reference to exemplary embodiments and the accompanying figures.

First exemplary embodiment: Production of a μc-Si: H absorber layer on a substrate at different rates of deposition

The substrate used was a glass TCO layer system (TCO), plus a microcrystalline p layer deposited thereon. The glass has a thickness of about 1 millimeter, the TCO layer has a thickness of about 500 nanometers, and the p-layer has a thickness of about 10 nanometers.

A plasma chamber system was selected in which the doped p- and n-layer and the undoped absorber layer were deposited in different chambers.

To prepare the μc-Si: H absorber layer deposition on the p-layer, the named substrate is first positioned parallel to the showerhead high-frequency electrode with an electrode spacing of 10 mm and heated in vacuo to a substrate temperature of 200 ° C. The base pressure is in this case 10 ^"7 mbar. Then, the plasma chamber can be coated up to a size of 30x30 cm ² in the substrates, continuously with a hydrogen flow rate of 4000 sccm (corresponding 444.4 sccm per 100 cm ² to be coated substrate surface) and a silane flux of 57 sscm at 1 nm / s deposition rate (this corresponds to 6.3 sccm per 100 cm ² of substrate surface to be coated) and 102 sccm of silane flow at 2.5 nm / s deposition rate (this corresponds to 11.3 sccm each 100 cm ² of substrate surface to be coated) flooded while the plasma chamber pressure continuously during the entire deposition process to 9.3 mbar regulated.

The gas mixture present in the chamber is completely discharged from the chamber. In this case, a constant deposition pressure is formed. The process gases are introduced into the plasma chamber through the showerhead radio frequency electrode. If the μc-Si: H absorber layer deposition takes place at a generator power of 800 W, then the silane concentration (= silane flow / (sum of silane flow and hydrogen flow)) is set to a value of 1.4%. For the deposition of the μc-Si: H absorber layer with a generator power of 1800 W, a silane concentration of 2.5% is set.

By means of the generator, a plasma is ignited between the substrate and the showerhead RF electrode and then regulated to a specific generator power. The plasma excitation frequency is 40.68 MHz. If a deposition rate of the μc-Si: H absorber layer of 1 nm / s is to be achieved, the generator power must be set to a value of 800 W (corresponds to 0.6 W / cm ² electrode area). To obtain a μc-Si: H deposition rate of 2.5 nm / s, a generator power of 1800 W (corresponding to 1.3 W / cm ² electrode area) must be set.

During the μc-Si: H absorber layer deposition, all specified process parameters remain unchanged. The deposition time required results from the desired μc-Si: H absorber layer thickness and the μc-Si: H deposition rate associated with the respective production process. The μc-Si: H absorber layer deposition ends when the generator is switched off after the set deposition time has elapsed.

Due to the set silane concentration (= silane flow / (sum of silane flow and hydrogen flow)), the μc-Si: H absorber layer is deposited close to the μc-Si: H / a-Si: H transition and therefore optimized with respect to the layer properties.

On the μc-Si: H absorber layer produced by this process, an n-doped layer was deposited to produce a μc-Si: H single solar cell. This has a thickness of about 20 nanometers. The back contact was made of ZnO / Ag (80 nanometers ZnO and 700 nanometers Ag). Depending on the μc-Si: H absorber layer thickness and the μc-Si: H deposition rate, the cell efficiency of this single cell reached the following values:

It becomes clear that lowering the absorber layer thickness to values around 500-600 nanometers (cells 3 to 4) does not lead to a reduction in cell efficiency due to a deposition rate increase from 1 nm / s to 2.5 nm / s. In contrast, in the control experiment with a prior art absorber layer thickness (standard: 1000-2000 nanometers for single cells, 1000-3000 nanometers for stacked solar cells), a significant loss of cell efficiency from 9.2 to 7.8 percent due to increased Layer thickness measurable. As a result datated the inventive method to a significant increase in the output of solar cells with otherwise unchanged efficiency.

Second exemplary embodiment: Production of a μc-Si: H absorber layer on a substrate at additionally high base pressure

A plasma chamber system was chosen in which the doped p- and n-layer and the undoped absorber layer were deposited in different chambers.

In the evacuated state, the plasma chamber has a base pressure of approximately 10 ^-8 mbar For purposefully increasing the oxygen content in the μc-Si: H absorber layers deposited there, an artificial air leak is attached to the chamber, the strength of which is controlled by a needle valve can be adjusted. Depending on how far the needle valve is open, the base pressure in the plasma chamber also increases. The base pressure acts as a measure of the strength of the air leak and thus the built-in amount of oxygen.

To prepare the μc-Si: H absorber layer deposition, the substrate is first positioned parallel to the showerhead RF electrode with an electrode spacing of 10 mm and heated in vacuum to a substrate temperature of 200 ° C. Thereafter, the plasma chamber, in which substrates can be coated to a size of 10x10 cm ² , is continuously flooded with a hydrogen flux of 360 sccm and a silane flow of 5.0 sccm (about 1.4% silane concentration) and the plasma chamber pressure at 13 , 3 mbar regulated.

The gas mixture present in the chamber is completely discharged from the chamber. In turn, a constant deposition pressure is formed. The process gases are admitted through the showerhead high-frequency electrode into the process chamber.

Using the generator, a plasma is ignited between the substrate and the showerhead RF electrode and then regulated to a generator power of about 60 W (corresponding to about 0.4 W / cm ² electrode area), so that a μc-Si: H deposition rate of 0.7 nm / s is achieved. The plasma excitation frequency is 13.56 MHz.

During the μc-Si: H absorber layer deposition, all specified process parameters remain unchanged. The required deposition time results from the desired μc-Si: H absorber layer thickness and the μc-Si: H deposition rate associated with the manufacturing process. The deposition of the μc-Si: H absorber layer ends when the generator is switched off after the set deposition time has elapsed.

On the μc-Si.Η absorber layer produced by this process, an n-doped layer was deposited to produce a μc-Si: H single solar cell. This has a thickness of about 20 nanometers. The back contact was made of Ag (700 nanometers). Depending on the base pressure set in the plasma chamber via the needle valve and the μc-Si: H absorber layer thickness, the cell efficiency reached the following values:

It is clear that result in a layer thickness of less than 1000 nanometers for the absorber layer (Examples 1-3), also a substantial increase of the base pressure to approximately 10 ^"5 mbar to tolerable efficiency losses of only 1.6%. This is still too Consider that Example 3 compared to Example 1 and Example 2 has a good 100 nanometers lower absorber layer thickness, which at this absorber layer thickness level is responsible for the loss of efficiency of 1, 6%.

In contrast, above 1000 nanometers layer thickness of the absorber layer already significantly increased efficiency losses with an increase in the base pressure of 10 ^"8 to 10 ^{" 5} mbar detectable. These are already at 2.7%. In the case that the absorber layer thickness is even about 3000 nanometers, the detectable loss of efficiency is again significantly increased. In this case, it is even 3.7% with a base pressure increase from 10 ^'8 to 10 ^{' 5} mbar.

As a result of these results, an additional effect has already occurred at a deposition rate of 0.7 nm / s which is slightly higher than the state of the art and at the same time thin layer thicknesses of here 400-530 nanometers of the deposited microcrystalline absorber layer. Thus, despite a relatively high base pressure of 10 ^"5 mbar, only a small efficiency loss detectable. This is absolutely intolerable for process technical point of view. The efficiency loss is significantly less than with the thick absorber layers of Examples 7-9.

Example 3 in Table 2 has, with a comparable to the first embodiment, a thick absorber layer of 530 nanometers and with a back contact made of ZnO / Ag corresponding to an efficiency of about 7%.

Third Exemplary Embodiment: Production of a μc-Si: H absorber layer for a tandem solar cell on a substrate

In stacked solar cells based on amorphous (a-Si: H) and microcrystalline (μc-Si: H) silicon, the total thickness of the active layers is essentially influenced by the thickness of the μc-Si: H absorber layers. An example of a-Si: H / μc-Si: H-based stacked solar cells are a-Si: H / μc-Si: H tandem solar cells, which consist of exactly one a-Si.Η top cell and one μc-Si: H bottom cell. The top cell is the cell into which the light first enters.

In the state of the art, the μc-Si: H absorber layer contributes about 75% of the total thickness of the active layer of an a-Si: H / μc-Si: H tandem solar cell.

Since, in particular, in an a-Si: H / μc-Si: H tandem solar cell, the μc-Si: H bottom absorber layer with a thickness of 1000-3000 nanometers has by far the largest single layer thickness, its deposition rate decisively determines the annually achievable output of a production plant ,

For producing a solar cell with pinpin structure for a tandem configuration is as a substrate, a glass TCO-a-SiiH top cell layer system (TCO Engl, transparent conductive oxide) plus deposited thereon microcrystalline p-layer. The glass has a thickness of approximately 1 millimeter, the TCO layer has a thickness of approximately 500 nanometers, and the a-Si: H top cell has an adapted thickness of each due to the series connection to the μc-Si: H bottom cell about 140-390 nanometers up. This means that, depending on the μc-Si: H absorber layer thickness, the absorber layer thickness of the a-Si: H top cell was adjusted so that both single cells supply the same current.

The deposited thereon microcrystalline p-layer has a thickness of about 10 nanometers.

To prepare the μc-Si: H absorber layer deposition, the substrate is first positioned parallel to the showerhead RF electrode with an electrode spacing of 10 mm and heated in vacuum to a substrate temperature of 200 ° C. The base pressure is from about 10 ^-7 mbar. Thereafter, the plasma chamber, in which substrates can be coated to a size of 30x30 cm ² , is continuously flooded with a hydrogen flux of 2700 sccm and silane flow of 24 sccm (0.9% silane concentration) and the plasma chamber pressure at 13.3 mbar regulated.

Using the generator, a plasma is ignited between the substrate and the showerhead high-frequency electrode and then regulated to a generator power of about 500 W (corresponds to about 0.4 W / cm ² electrode surface), so that a μc-Si: H deposition rate. te of 0.7 nm / s is achieved. The plasma excitation frequency is 13.56 MHz.

During the μc-Si: H absorber layer deposition, all specified process parameters remain unchanged.

The deposition time required results from the desired μc-Si: H absorber layer thickness and the μc-Si: H deposition rate associated with the respective production process. The μc-Si: H absorber layer deposition ends when the generator is switched off Expiration of the set deposition time.

On the μc-Si: H absorber layer produced by this process, a microcrystalline n-type layer was deposited to produce the a-Si: H / μc-Si: H tandem solar cell. This has a thickness of about 20 nanometers. The back contact was made of ZnO / Ag (80 nanometers ZnO and 700 nanometers Ag). The a-Si: H / μc-Si: H tandem solar cell thus has a μc-Si: H bottom cell on an a-Si: H top cell.

After determining the initial cell efficiency, the a-Si: H / μc-Si: H tandem solar cells were then exposed in a degradation experiment for 1000 hours at a cell temperature of 50 ° C to an irradiance of 100 mW / cm ^{2 in} order to determine the stabilized cell efficiency , The following values resulted:

It is clear that the relative degradation-related loss of efficiency is lower in cells with increasingly low absorber layer thickness. While in Example 1 and 1850 nanometer layer thickness, this relative loss is still 18.5%, it is only 6.7% in Example 4 and a layer thickness of 440 nanometers. For tandem solar cells, the light-induced degradation is almost exclusively caused by a degradation of the a-Si: H top solar cell.

The consequence of the results of the embodiments is that the production cost optimum to significantly lower total layer thicknesses <1000 nanometers at high deposition rates> 0.5 nm / s and optionally high base pressure with correspondingly high Sauserstoffendkonzentration in the finished solar cell shifts. Thus, the a-Si: H / μc-Si: H tandem solar cell no. 4 has a total layer thickness of the active semiconductor layers (pinpin without front and back contact) of about 640 nanometers.

Claims

Patent claims

A method of depositing microcrystalline silicon on a substrate in a plasma chamber system comprising the steps of:

the plasma chamber system has a reactive, silicon-containing gas and hydrogen or exclusively hydrogen before the start of the plasma, the plasma is started,

- The chamber system is continuously supplied only reactive, silicon-containing gas after the plasma discharge or the chamber system is continuously fed to the plasma after a mixture comprising a reactive, silicon-containing gas and hydrogen, wherein the concentration of reactive, silicon-containing gas in the supply to the chamber greater is set as 0.5%, and

the plasma power is set between 0.1 and 2.5 W / cm ² electrode surface,

a deposition rate of greater than 0.5 nm / s is selected, and the microcrystalline layer having a thickness of less than 1000 nanometers is deposited on the substrate.

2. The method of claim 1, wherein the flows of the chamber supplied and derived from the chamber gases or gas mixtures are controlled so that forms a constant deposition pressure during the process.

3. The method according to any one of the preceding claims, wherein a deposition rate up to 5.0 nm / s, in particular between 1.0 to 2.5 nm / s is selected.

4. The method according to any one of the preceding claims, wherein a microcrystalline layer having a thickness of 200 to 800 nm, in particular from 400 to 600 nm is deposited.

5. The method according to any one of the preceding claims, characterized in that an excitation frequency of 13.56 to about 100 MHz at an electrode spacing of 5 to 25 millimeters, in particular an electrode spacing of 10 to 25 millimeters is selected.

6. The method according to any one of the preceding claims, characterized in that the deposition pressure in the plasma chamber between 1 and 25 mbar is set.

7. The method according to any one of the preceding claims, wherein the chamber after the plasma discharge continuously at least exclusively reactive, silicon-containing gas in a flow of 0.5 sccm up to 20 sccm / 100 cm ² coating area, in particular from 0.5 sccm to 10 sccm / 100 cm ² coating area.

8. The method according to any one of the preceding claims, characterized in that the base pressure in the plasma chamber system is at least 10 ^"6 mbar, in particular at least 10 ^{" 5} mbar.

9. The method according to any one of the preceding claims, characterized in that a substrate temperature between 100 to 350 ⁰ C is selected.

10. The method according to any one of the preceding claims, characterized in that at the same time present in the chamber gas mixture at least partially from the

Chamber is derived.

11. The method according to any one of the preceding claims, characterized in that the substrate is an electrical contact layer and then deposited microcrystalline n-layer or a glass TCO-a-Si: H cell with microcrystalline p-layer deposited thereon or a metal TCO -a-Si.Η cell with microcrystalline p-layer deposited thereon.

12. solar cell with at least one pin structure or at least one nip structure, prepared according to one of the preceding claims.

13. Solar cell according to claim 12, characterized in that the microcrystalline Absorberberschicht has an oxygen content of more than 2 * 10 ¹⁹ oxygen atoms / cm ³ .

14. Solar cell according to claim 12 or 13, characterized in that the solar cell has an efficiency of at least 7-8%.

15. Solar cell according to one of the preceding claims 12 to 14, characterized in that the solar cell is a a-Si: H / μc-Si: H based stacked solar cell.

16. Solar cell according to the preceding claim, characterized by a total layer thickness of all active semiconductor layers of less than 1000 nanometers.